Protective elements in which a heat-generating member and a low-melting metal member are layered or disposed in the same plane on a substrate are known as protective elements that can be used to prevent not only over-current but also overvoltage, and which are useful in secondary cells for portable electronic devices and so forth (Japanese Patent No. 2,790,433, Japanese Patent Application Laid-Open No. H10-116549). With this type of protective elements, in the event of a malfunction, current flows to the heat-generating member, and the heat-generating member generates heat, which blows out the low-melting metal member.
The increasing performance of portable electronic devices in recent years has required the above protective elements to have higher rated current. One way to raise the rated current of a protective element is to increase the thickness or width of the low-melting metal member and thereby increase its cross sectional area and lower its resistance. Unfortunately, the problem with increasing the cross sectional area of the low-melting metal member is that it results in a longer operating time needed to block off current in the event of overcurrent or overvoltage. Moreover, increasing the thickness of the low-melting metal member goes against the need to make elements thinner.
Yet another problem with the above-mentioned protective elements was the variance in the time it took for the low-melting metal member to go from a molten state to being blown out by the heat generated by the heat-generating member, and it has been proposed to set a specific relationship between the low-melting metal member and the blow-out effective electrode surface area (Japanese Patent Application Laid-Open No. 2001-325869).
It is an object of the present invention to provide a protective element comprising a heat-generating member and a low-melting metal member on a substrate, in which the low-melting metal member is blown out by the heat generated by the heat-generating member, wherein the operating time is shortened even when the sectional area of the low-melting metal member has been increased in order to raise the rated current, and the time from heat generation of the heat-generating member up to blow-out is more consistent.